Two cities, one in the United States and the other in Canada, over 700 miles apart, were experiencing nearly identical problems caused by manual addition of powdered activated carbon to their municipal water treatment systems. Both cities not only eliminated a health hazard caused by carbon powder dust, but also reduced the amount of labor required to add powdered activated carbon to the system by installing a bulk bag unloading system.
The city of Swift Current draws its municipal water supply from a reservoir formed by a dam on the Swift Current Creek at an average rate of 1.5 million gal a day with a peak rate of 5.5 million gal a day. Potassium permanganate is added at a concentration of 1.5 parts per million (ppm), then powdered activated carbon is added in the form of a slurry consisting of 0.13–0.47% solids, and the water is gravity fed into a low well at a rate of approximately 25 gal/min. Reaching a final concentration of 20 ppm, the carbon’s function is to adsorb organic materials, eliminating unwanted tastes and odors.
After addition of liquid alum, sludge conditioners, and polymers in the clarifier, filtered water is pumped to a clear well located at the water treatment facility with a capacity of 180,000 gal and into two reservoirs within the city itself, each holding 1.5 million gal, for distribution to residents of the city of Swift Current.
The city of Lafayette draws its water from two local reservoirs formed by dams on the Boulder Creek and from one reservoir on the South Boulder Creek. Consumption is approximately 2 million gal per day during the winter, but can reach as much as 10 million gal per day during the summer months. From the reservoirs, water flows into an on-site aeration and flocculation facility similar to that used by the city of Swift Current, where powdered activated carbon and other treatment chemicals are added. After final filtration, the water is pumped into three storage tanks, which together can hold up to 13 million gal of water, for distribution to residents of the city of Lafayette.
Powdered Activated Carbon Added Manually
|A Spout-Lock clamp ring creates a high-integrity seal between the bag spout and a Tele-Tube telescoping tube that maintains constant downward tension on the bag, promoting complete, dust-free discharge.|
Prior to the installation of the bulk bag unloading systems, operators at both facilities were emptying 35–45-lb bags of powdered activated carbon, generating carbon dust and putting physical strain on workers.
Powdered activated carbon is an extremely fine powder with an average particle size of only 20 μm and a bulk density of 21.5 lb/cu ft. “The slightest air movement causes carbon dust everywhere, and it cakes and smears on everything it touches, including your clothes and your skin,” says Rudy Holland, city of Swift Current superintendent of water treatment. “We tried using a dust collection system, but it never worked properly. Also, the capacity of the system was limited by the small screw feeder, which took 16 hours to transfer the powdered carbon from the 1000-lbcapacity hopper into the liquid slurry eductor. Since the water treatment facility operates only one shift a day— staffed for 8½ hours, then unmanned and controlled by computer for 15½ hours—the hopper, if not completely filled at the beginning of the unmanned shift, could potentially run out of carbon when personnel were not present to refill it.””
City of Swift Current operators were opening and emptying as many as twenty-five 45–55-lb bags of powdered activated carbon a day into the hopper. City of Lafayette operators were carrying a 35–40-lb bag of powdered carbon up a set of metal stairs and emptying it directly into the aeration and flocculation facility. “This not only created hazardous carbon dust, but since we used about 2–4 bags a day during the winter and as many as 10–17 bags a day during the summer, it also required extensive manual labor and placed operators working alone on the midnight shift in danger of falling and injuring themselves," explains Ed Zimbleman, lead operator. "In addition, bags would occasionally break, requiring operators to hose down the entire area."”
Bulk Bag Unloading and Feeding Systems Eliminate Hazards
|The airtight system features a side-mounted Bag-Vac dust collector that draws displaced air and dust from the hopper and collapses empty bags dust-free.|
|Upper level: bulk bag discharger with control panel and Bag-Vac dust collector|
At both facilities, installation of an automated bulk bag unloading and feeding system eliminated dust caused by untying, discharging, collapsing, and removing individual small bags, while increasing operator safety and reducing manual labor.
The city of Swift Current now purchases 1100-lb bags and unloads 1–3½ bags per week. The city of Lafayette unloads between one 900-lb bulk bag per week during the winter and as many as 4½ bags per week during the peak summer months.
The Flexicon bulk bag dischargers incorporate devices that contain dust and promote complete discharge. A manual Spout-Lock clamp ring is raised pneumatically by a Tele-Tube telescoping tube, allowing an operator to make a high-integrity, dust-tight connection with the bag spout. The telescoping tube is then released to exert continuous downward tension on the clamp ring, and in turn the bag spout, which elongates the bag as it empties to promote complete discharge.
|Lower level: Frame holds 5 cu-ft-capacity receiving hopper from which a volumetric feeder meters powder carbon to a washdown hopper and liquid slurry educator. Here the carbon blends with fresh water (green tube) flowing to the clarifier.|
Flow is additionally promoted by Flow Flexer pneumatically actuated plates that raise and lower opposite bottom edges of the bag, causing the activated carbon to flow into and through the bag spout.
A Power Cincher flow control valve encircles the upper portion of the bag spout, allowing gradual discharge once the drawstring is untied, as well as retying of partially empty bags.
Mounted on the side of both unloading frames is a Bag-Vac dust collector that draws displaced air and dust from a hopper vent during discharge, and collapses empty bags dust-free, ready for tie-off and removal.
Two Systems Have Important Differences
|Bulk bag is suspended from a lifting frame that rests in receiving cups atop each of four frame posts. Pneumatically-actuated Flow-Flexer plates beneath the left and right bottom edges of the bag hinge up and down at timed intervals to loosen compacted materials and raise the bottom of the bag into a steep V shape, promoting total discharge through the bag spout.|
At the city of Swift Current, a hoist from a 24-ft-high ceiling-mounted monorail loads an 1100-lb bag onto a 5-ft-high half-frame bulk bag discharger mounted on a 6½-ft-high platform. The discharger frame is integrated with a 10-ft-high frame that holds a 2000-lb-capacity hopper (for the contents of two bags), a twinscrew volumetric feeder, washdown hopper, and liquid slurry eductor. The powdered activated carbon flows from the bulk bag to the hopper and then to the feeder, which meters it into the washdown hopper and eductor to blend with the 25 gal/min water streaming toward the low well. A control panel regulates the dust collector, the bag activators, feeder, and flow-promoting air pads on the side of the hopper.
Forklift Loading Replaces Manual Emptying of Bags
A city of Lafayette operator forklift loads a bag-lifting frame onto bulk bag discharger.
At the city of Lafayette an operator forkliftloads a 900-lb bag in a lifting frame onto a full-height 9¾-ft-high discharger. The powdered carbon flows through the telescoping tube and 2-ft-long downspouting that extends through the floor to a 5-cu-ft-capacity receiving hopper contained in a 7-ft-high frame, which also holds the twin-screw volumetric feeder, washdown hopper, and liquid slurry eductor. Controls govern the dust collector, feeder, and air fluidizers for hopper flow, and notify the operator to change empty bags.
Continuous blending of powdered activated carbon from the volumetric feeder with fresh water creates a slurry, consisting of as little as 0.006% solids in the winter to as much as 0.05% solids in the summer. It is added to the clarifier at a rate of 120 gal/min, resulting in a final carbon concentration of 5 to 7 ppm. “We basically just set it and forget it,” says Zimbleman.
Design and Installation a Team Effort
|Operator inspects powdered carbon entering the washdown hopper.|
The City of Swift Current system was design-ed by a local consultant in Regina, SK, with the help of Mequipco Ltd., who supplied the equipment, and was installed by a local Saskatchewan contractor. Mequipco Ltd. (www.mequipco.com) is a manufacturer’s representative, equipment supplier, and systems integrator based in Calgary, AL, Canada whose primary focus is on mechanical equipment for water and wastewater treatment. Dan Landry and Kevin Dickinson headed the Mequipco team that assisted with the system startup and design.
|Bottom frame consolidates a receiving hopper, volumetric feeder, washdown hopper, and liquid slurry eductor.|
|Water flows from the sedimentation basin, through the powdered carbon addition system, and into the distribution system.|
The City of Lafayette system was designed and installed by Flexicon Corp. (www.flexicon. com) and Process Control Equipment Co. (www.pcecompany.com), a manufacturer’s representative firm headquartered in Lehi, UT, specializing in material handling equipment for powder and granule processing. Susan Aberle of the company’s Wheat Ridge, CO regional office assisted with installation and startup.
““We evaluated two other machines in addition to the Flexicon system,” says Zimbleman. “Although they were similar, the other two manufacturers were unwilling to make the necessary physical modifications to our facility, including cutting a hole in a concrete floor in order to put the hopper on top of the feeder.””
|At City of Swift Current, a bag is loaded onto a half-frame bulk bag discharger from a ceiling-mounted monorail. The frame below the discharger incorporates a 2000-lb-capacity hopper, volumetric feeder, washdown hopper, and a liquid slurry eductor that draws carbon powder into a water stream.|
Several modifications were made after the system was installed. “We were having problems with the washdown hopper clogging from time to time,” Zimbleman explains. “The hopper is 2 ft high and 2 ft in diameter at the top, but only 6 in. in diameter at the bottom. Susan and her team added jets with plastic nipples so the water circulated around the hopper instead of just spraying into it, which eliminated the problem.””
Flexicon Corp. (Bethlehem, PA) is world a leader in the manufacture design and of bulk handling equipment and custom-engineered and plant-wide integrated systems transport, that discharge, fill, weigh, blend, and deliver, feed a broad range of powder and bulk solid materials. For more information, visit www.flexicon.com.